Emphysema is defined as abnormal permanent enlargement of a lung's airspaces distal to the terminal bronchioles, accompanied by destruction of their walls, without obvious fibrosis. When x rays are applied to a lung, the changes to the lung associated with emphysema decrease the attenuation of x rays passing through the thorax and shift their distribution, allowing advanced emphysema to be detected during life. Not only is it important to be able to detect emphysema during life, it is also important to determine the extent and severity of emphysema during life.
The ability to estimate the extent and severity of emphysema during life is important for several reasons. The accurate detection of lung destruction when it first appears and the careful mapping of its progression over time allow the natural history of emphysema to be better understood. Further, the treatment of advanced emphysema by lung volume reduction surgery requires knowledge of the location of the lesions and an objective method of assessing the surgical result. Finally, recent provocative experimental studies, suggesting that alveolar number and surface area to volume ratio can be restored to a normal level by pharmacotherapy in rats with elastase-induced emphysema ("Retinoic acid treatment abrogates elastase-induced pulmonary emphysema in rats." Massaro, G. et al., Nature Medicine, 3:675-677 (1997)), indicate a future need for measurements that can accurately assess the effectiveness of such therapeutic interventions.
A need exists for methods and apparatus for determining certain parameters of a lung, such as lung surface area, that can be used to diagnose and monitor emphysema progression in a patient, both before and after the patient is surgically and medically treated.